The attacks began shortly after President Trump’s election in November and it wasn’t until spies posted at the US Embassy in Havana under diplomatic cover began reporting bizarre sounds and experiencing strange physical effects that officials finally realized something was wrong, the Associated Press reported.

The Trump administration described the 21 victims as US Embassy personnel or “members of the diplomatic community,” which suggests they were diplomats and family members, and provided no motive except for an attempt to disrupt US-Cuban relations.

But investigators working behind the scenes immediately began looking at sinister motives in the worlds of spycraft and espionage because many of the first cases of the noise attacks were reported by intelligence workers posted at the embassy, the Associated Press reported, citing a half-dozen officials.

The first reports of the piercing, high-pitched noises led officials to believe the US intelligence on the island were being targeted, but they soon learned that diplomats had also been victims of similar attacks.

The US spies among the 21 cases said they heard sounds, described as like loud crickets, inside and sometimes outside their homes in Havana and began feeling ill. They suffered brain injuries and hearing loss.

But then the attacks became more sophisticated.

In some of the most recent cases, the victims said they hadn’t heard any noises and were unaware the attacks were even happening. They only reported feeling the symptoms later.

US officials have not been able to determine the device responsible and cannot say whether the attacks have ended.

Last week, the State Department warned Americans to stay out of Cuba, saying that while no citizens have been affected, the attacks “have occurred in US diplomatic residences and hotels frequented by US citizens.”

The US is also instructing embassy personnel what to watch and listen for to help identify an ongoing attack and have created a map of Havana where the incidents have happened.